U.S. patent number 6,744,052 [Application Number 09/889,851] was granted by the patent office on 2004-06-01 for x-ray pixel detector device and fabrication method.
Invention is credited to Christer Frojdh, Jan Linnros, Sture Petersson.
United States Patent |
6,744,052 |
Petersson , et al. |
June 1, 2004 |
X-ray pixel detector device and fabrication method
Abstract
A method and device for producing an X-ray pixel detector, for
X-ray photons, the detector presenting high efficiency combined
with high resolution for obtaining a high image quality detector
while at the same time minimizing the X-ray dose used. The
application is particularly important whenever the X-ray photon
absorption distance is much longer than the required pixel size.
The arrangement presents a structure based on light-guiding of
secondarily produced photons within a scintillating pixel detector
in conjuction with, a CCD or a CMOS pixel detector. The structure
presents a matrix (8) having deep pores (10) fabricated by
high-aspect ratio silicon etching techniques producing very thin
walls and with a pore spacing less or equal to the size of a pixel
(2) of the image detector used. The pore matrix is subsequently
filled by melting a scintillating material into the pores such
that, in each pore, a single scintillating block is formed. The
silicon matrix (8) may further utilize a reflective layer to
increase light guiding down to the image detector chip.
Inventors: |
Petersson; Sture (Uppsala,
SE), Linnros; Jan (Bromma, SE), Frojdh;
Christer (Sundsvall, SE) |
Family
ID: |
20414176 |
Appl.
No.: |
09/889,851 |
Filed: |
October 17, 2001 |
PCT
Filed: |
January 21, 2000 |
PCT No.: |
PCT/SE00/00135 |
PCT
Pub. No.: |
WO00/43810 |
PCT
Pub. Date: |
July 27, 2000 |
Foreign Application Priority Data
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Jan 21, 1999 [SE] |
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9900181 |
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Current U.S.
Class: |
250/370.11;
250/361R |
Current CPC
Class: |
G01T
1/2018 (20130101) |
Current International
Class: |
G01T
1/00 (20060101); G01T 1/20 (20060101); G01T
001/20 () |
Field of
Search: |
;250/361R,367,368,483.1,370.11,458.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Hannaher; Constantine
Assistant Examiner: Moran; Timothy
Attorney, Agent or Firm: Young & Thompson
Claims
What is claimed is:
1. A method for fabricating a structured high resolution
scintillating device based on light guiding of secondary produced
scintillating photons for use in an X-ray pixel detector device
with an image detector chip (1), characterized by the steps of
fabrication of a silicon pore matrix (8) presenting a pore spacing
(10) corresponding to the image detector pixel size (2), by
utilizing silicon etching techniques such as deep reactive ion
etching, electrochemical techniques or other techniques providing
high-aspect ratios such that thin pore walls of thickness reaching
down to a few micrometers will be maintained for an active
detection area optimization; using the silicon pore matrix (8) as a
mold when melting a scintillator material into the pores to form in
each pore a single scintillating block in order to eliminate
grain-boundary scattering of scintillating photons.
2. The method according to claim 1, characterized by the further
step of, after etching but before molding, depositing a metallic
reflective layer in the pores.
3. A method of fabricating a high resolution scintillating device
for an X-ray pixel detector, comprising the steps of: fabricating a
silicon pore matrix having plural pores corresponding to locations
of pixels in the X-ray pixel detector, the plural pores being
formed by etching a silicon substrate; melting a scintillating
material into the plural pores of the silicon pore matrix to form
in each of the plural pores a single scintillating block; and
providing, after the fabricating step but before the melting step,
a reflection layer for light guiding by oxidation of the silicon
pore matrix or by deposition of any layer having a resulting
refractive index being lower than that of the scintillating
material.
4. A scintillating device for simultaneously maintaining resolution
and increased sensitivity for X-ray radiation in an imaging
arrangement, characterized by utilization of a fabrication method
producing a silicon pore matrix (8) presenting a pore spacing (10)
corresponding to an image detector pixel size (2), the pore matrix
having deep pores (10) presenting thin walls of a thickness
reaching down to a few micrometers to create a pore spacing
corresponding to the pixel size (2) of an image detector chip (1),
the pore matrix (8) further containing scintillating material which
is melted into the pores (10) to form in each pore a single
scintillating block in order to eliminate grain-boundary scattering
of scintillating photons.
5. The device according to claim 4, characterized by a reflective
layer (12) onto the thin walls of the matrix to increase light
guiding down to the image detector chip (1).
6. The device according to claim 4, further comprising a reflection
layer on walls of the pores, the reflection layer being one of an
oxidation of the silicon pore matrix and a layer having a
refractive index lower than a refractive index of the scintillating
material.
7. A method of fabricating a high resolution scintillating device
for an X-ray pixel detector, comprising the steps of: forming
plural pores in a silicon substrate to form a silicon pore matrix;
and melting a scintillating material into the plural pores of the
silicon pore matrix to form a scintillating block in each of the
plural pores.
8. The method of claim 7, further comprising the step of providing
a reflection layer on walls of the pores by oxidizing the silicon
pore matrix in the pores.
9. The method of claim 7, further comprising the step of providing
a reflection layer on walls of the pores, the reflection layer
having a refractive index lower than that of the scintillating
material.
10. The method of claim 7, wherein the plural pores correspond to
locations of pixels in the X-ray pixel detector.
11. The method of claim 7, wherein the plural pores are spaced more
closely than pixels in the X-ray pixel detector.
Description
TECHNICAL FIELD
The present invention relates to an X-ray pixel detector, and more
exactly to a pixel-camera based i g detector for X-ray photons with
high efficiency combined with high resolution.
BACKGROUND
Silicon devices as CCDs and CMOS pixel detectors are frequently
used for X-ray imaging. Due to the low stopping for X-rays in
silicon, the detector is generally coated with a scintillating
layer. When using scintillating layers for imaging there is a
trade-off between quantum efficiency and resolution. In order to
get high quantum efficiency for X-rays the layer should be made
thick, but that will reduce the spatial resolution in the image.
The quantum efficiency for X-rays is one of the most critical
parameters for medical X-ray imaging devices since the signal to
noise ratio in the image depends on the number of X-ray photons
contributing to the image. Since photoelectric absorption is a
single event an X-ray photon will either be fully absorbed or pass
unnoticed through the detector.
X-ray generators for dental X-ray imaging operate with an
accelerating voltage of 60-90 kV giving mean photon energy in the
range 30-40 keV. The material thickness required to stop 80% of the
X-ray photons is in the range 150-500 .mu.m for the commonly used
scintillators. The primary interaction between the photon and the
material, photoelectric absorption, is a single event. The light in
the scintillator is then generated by a large number of secondary
reactions taking place within a few microns from the location of
the primary interaction. As a result a flash of light is generated
close to the spot of the primary interaction and radiated in all
directions. The quantum efficiency for X-rays is then related to
the probability for the primary interaction to occur and to a very
small extent to the secondary interactions. In the energy range of
interest for such an application and with the materials used as
scintillators the primary interaction is generally a photoelectric
absorption. Compton scattering and other events are less likely to
occur.
The light generated in the scintillator is projected onto the
sensor with a spot size, which is proportional to the distance
between the point of interaction and the position of absorption in
the sensor. The projection is also affected by the refractive
indexes of the materials the beam will pass. For a typical
combination of scintillator and CCD, the scintillator thickness
should be less than 100 .mu.m to achieve a spatial resolution>10
line-pairs/mm, as required for dental X-ray imaging.
A method to improve the spatial resolution of thick scintillating
layers is to define pixels in the scintillator, as proposed in
EP-A2-0 534 683, U.S. Pat. No. 5,059,800 and U.S. Pat. No.
5,831,269 and to make that the light generated within one pixel is
confined within that pixel. Pixel definition in scintillators can
be done in a number of ways, e.g. columnar growth of scintillator
crystal or groove etching in scintillating films. In EP-A2-0 534
683 dicing or cutting is suggested for separating scintillator
elements from a large scintillator block, as appropriate for larger
lateral dimensions.
The method for columnar growth of scintillating crystals is well
known. It has been used to grow CsI for many years. The document
WO93/03496 discloses for instance growth of separate columns in
different scintillators whereas in U.S. Pat. No. 4,663,187 a
scintillator is held close to the melting point resulting in the
formation of domains. The disadvantage of techniques for growth of
separated columns is that the columns tend to grow together for
thick layers and that light will leak to adjacent columns. It is
difficult to apply a light reflector between the columns.
Etching of grooves in scintillating materials is considered to be
extremely difficult due to the high aspect ratios required by the
application. With a pixel size of 50 .mu.m and an allowed area loss
of less than 20% the groove width should be less than 5 .mu.m. If
the film thickness is 200 .mu.m the aspect ratio will be 40. This
aspect ratio can only be realised by advanced silicon processing
techniques whereas etching techniques for scintillating materials
are far less developed. Nevertheless, U.S. Pat. No. 5,519,227
claims that laser-based micro-machining techniques could be used to
define narrow grooves in a scintillating substrate. However, the
technique is inherently slow as the laser needs to be scanned
several times in every groove. Furthermore, it is not clear whether
re-deposition onto the walls will occur as a result of this laser
ablation, which could potentially block a narrow groove.
Summarising, various techniques have been proposed for the
fabrication of a scintillator array that would provide light
guiding of secondary photons to an underlying imaging detector,
These techniques are all restricted in one or several aspects:
either too large lateral dimensions (cutting, dicing), problems of
forming a well-defined narrow wall (laser ablation), cross talk
between adjacent pixels (columnar growth technique) or a lengthy
processing time (valid for most of these techniques). Finally,
deposition of a reflective layer in the grooves is usually
suggested to improve light guiding and reduce cross talk. But, none
of these fabrication schemes have proposed a detailed scheme how
the reflective layer would be produced. This is not an easy task
considering the narrow pore geometry and materials involved.
Therefore there is still a desire to develop a device and it's
associated fabrication method, which should be able to handle thick
scintillating material layers but with a maintained resolution
which corresponds to the individual pixel size. Furthermore, the
fabrication technique should preferably be fast, as for a mass
scale production type, and relying as much as possible on existing
processes and machinery.
SUMMARY
The objective of the present invention is to design and develop a
fabrication method for an X-ray pixel detector, i.e. an imaging
detector for X-ray photons presenting high efficiency combined with
high resolution to obtain a high image quality detector while at
the same time minimizing the X-ray dose used. The application is
particularly important whenever the X-ray photon absorption
distance is much longer than the required pixel size.
It is proposed to take advantage of the mature processing tools of
the silicon microelectronics technology where lateral dimensions on
a micrometer scale may readily be achieved. Thus, a silicon mold is
fabricated by high-aspect ratio etching of a silicon substrate for
form an array of pores. This array is subsequently oxidized to
provide a low refractive index layer in contact with each
individual scintillator block, formed by melting a scintillating
material into the pores.
A scintillator device according to the present invention presents a
structure based on light guiding of secondarily produced
scintillating photons in a pixel detector in conjunction with, for
instance, a CCD or a CMOS pixel detector. The structure according
to the invention presents a matrix having deep pores created by
thin walls presenting a pore spacing appropriate to the image
detector in use, and may utilize a reflective layer on the walls of
the matrix to increase light guiding down to the image detector
chip.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention, together with further objects and advantages
thereof, may best be understood by making reference to the
following description taken together with the accompanying
drawings, in which:
FIG. 1 illustrates a silicon CCD pixel detector for direct
irradiation by X-rays;
FIG. 2 illustrates a pixel detector as of FIG. 1 but provided with
a thin scintillator for increasing its efficiency for X-ray
radiation;
FIG. 3 illustrates a pixel detector as of FIG. 1 provided with a
thick scintillator for further increasing the efficiency for X-ray
radiation, but then loosing resolution;
FIG. 4 illustrates a CCD pixel detector using a thick pixel
scintillator residing inside pores formed in a matrix material
according to the present invention for maximum sensitivity and
maintained resolution;
FIG. 5 is a more detailed view of the structure forming pores for
increasing the efficiency of a CCD pixel detector; and
FIG. 6 is an enlargement of a portion of a pore indicating an extra
layer of silicon oxide for improving the wall reflecting
properties.
DETAILED DESCRIPTION
General Features
The most developed etching techniques exist for silicon processing.
According to the present application a grid is created by etching
rectangular holes in a silicon wafer. The holes can be etched to a
certain depth or go all the way through the wafer. The holes are
then filled with scintillating material.
The performance of such a device strongly depends on how well the
holes are filled, the transparency of the scintillator and the
reflection properties of the walls of the hole.
The present X-ray pixel detector concept is for clarity compared to
existing technology demonstrated in FIGS. 1 to 4. FIG. 1: A
standard silicon CCD arrangement has a very low efficiency for
X-ray photon detection, normally of the order of a few per cent.
This is because the penetration depth of X-ray photons, at energies
of the order 40 keV, is of the order of 1 cm in silicon and thus
the fraction absorbed within the active CCD layer is small
The efficiency will preferably be increased significantly by using
a scintillating material emitting a large number of visible photons
for every absorbed X-ray photon as is indicated in FIG. 2. Typical
absorption lengths for X-ray photons, at energies of the order 40
keV, are several 100 .mu.m. As already mentioned a layer of the
order 300 .mu.m of CsI is needed to absorb about 80% of the X-ray
photons. Thus, for thick scintillating films as indicated in FIG.
3, almost al X-ray quanta may be absorbed, which results in a high
efficiency detector. However, the trade-off is resolution, which
becomes much worse as the scintillator emits photons isotropically,
such that nearby pixels will also detect a significant number of
photons. An alternative route is to use a thin scintillating film
(of about same thickness as a pixel size) as indicated in FIG. 2,
but at the expense of a much lower efficiency.
Finally, in FIG. 4 is shown the concept of the invention resulting
in both high efficiency and high resolution. Here, a thick
scintillator is used which has been patterned into pixels
corresponding to the size of the pixels of the detector, e.g. a
CCD, in such a way that the scintillator pixels also serve as light
guides which confine the emitted photons to the same pixel element
only. Thus, no cross talk between pixels takes place and, depending
on the pixel thickness (length perpendicular to the CCD sauce) up
to 100% of the incoming X-ray photons may be absorbed. However, in
order to achieve a large effective detection area the spacing
between pixels must be short, e.g. for a typical 44 .mu.m pixel
size a 4 .mu.m gap between pixels results in -82% efficiency due to
the `dead area` in between pixels. Clearly, to minimize cross-talk
pixels may be reflection coated or the medium in between should be
highly absorbing.
The fabrication of pixels having a thickness of 300 .mu.m and a gap
of about 4 .mu.m from a scintillating material is not an easy task.
The present invention therefore benefits from the mature silicon
process technology using a silicon matrix wherein corresponding
pores have been fabricated and successively filled with a
scintillator material. The fabrication technology involves more or
less standard silicon fabrication technologies such as Deep
Reactive Ion Etching (DRIE), oxidation and/or metallisation. A
schematic drawing of the structure is shown in FIG. 5 where 3
pixels are displayed together with a close-up of the wall structure
between adjacent pixels being demonstrated in FIG. 6. In essence,
the structure contains three different materials to provide the
light-guiding effect the processing of which is accomplished one
after the other:
Silicon Pore Matrix
The silicon pore matrix of the present application may be
fabricated using two different techniques: Deep Reactive Ion
Etching (DRIE) or Electrochemical etching. DRIE is now an
established technique and several hundred .mu.m deep pores may be
fabricated. It has been found that it is possible to make, for
instance, 40.times.40 .mu.m square-formed pores with a wall
thickness of 3-4 .mu.m (representing -80% active area) and with a
depth of a few hundred .mu.m. A similar structure may be formed by
electrochemical etching of silicon starting from pore initiation
cones made by conventional lithography and non-isotropic
etching.
Wall Reflection Layer
Scintillating materials usually have an index of refraction (for
CsI n=1.79) which is significantly lower than that of silicon
(n=3.4). Thus, the major fraction of scintillating photons
impinging on the pore walls will penetrate into the silicon (Si)
matrix unless some reflection coating of the pore walls has been
provided. Therefore, this simple structure will have much lower
efficiency since no light guiding exists. In the silicon matrix the
light will be quickly absorbed due to the high absorption
coefficient for visible light in silicon. However, note that this
is a clear advantage of the present invention, as opposed to
several of the structures cited in the Background paragraph, as it
totally eliminates any cross talk between pixels.
To provide light guiding a reflecting layer must be introduced at
the walls. This may be accomplished either by oxidation or by
coating with a metal layer. Whereas silicon dioxide is much more
stable during subsequent processing, metal coating provides better
reflection. In the case of an oxide, a total reflection results
whenever the entrance angle is larger than the result of the
expression arcsin(n2/n1), where n2 and n1 represents a respective
refractive index The reflection results in a light-guiding cone
propagating upwards and downwards in the pore, see FIG. 5. The
difference to a metal-coated pore (where all light would be guided
in the pore) is, however, not that large as light rays impinging on
the walls close to normal incidence correspond to very long path
lengths before reaching the image detector cell and thus absorption
is more likely.
Finally, a reflecting layer at the bottom of the pore (or at the
top surface for a pore structure, which is transparent) is
desirable in order to redirect and collect photons emitted in the
upward direction.
Filling With Scintillating Material
Filling of the pores with scintillating material is a crucial step.
Extensive tests have proved that filling of the pores with
scintillating powder without melting does not yield an operational
device structure. This is because grain boundary scattering of the
light results in a very short penetration distance. An
index-matched fluid could possibly circumvent this problem but the
low effective density of the scintillator powder (large unfilled
fraction) would then demand very deep pores.
Due to this fact our invention involves melting of the
scintillating material to form single or polycrystalline blocks of
scintillator material within each pore. For this purpose we have
used CsI as a suitable material as it does not decompose upon
melting. The melting and filling should be carried out in a vacuum
to reduce problems with air bubbles in the pores, which
significantly affects efficiency and the light guiding ability of
the pores.
In summary, the present invention is based upon light guiding of
secondarily produced scintillating photons in a pixel detector in
conjunction with, for instance a CCD camera or a corresponding
device. The three ingredients of the preferred embodiment of the
structure are:
a) A matrix with deep pores, thin walls and a pore spacing
appropriate to the image detector chip in use
b) A reflective layer on the walls to increase light guiding down
to the image detector chip
c) A suitable scintillating material which is melted into the pores
to form a single scintillating block in order to eliminate
grain-boundary scattering
In addition, the invention concerns a suitable fabrication method
to this structure in an efficient way suitable for mass
production.
It will be understood by those skilled in the art that various
modifications and changes may be made to the present invention
without departure from the scope thereof, which is defined by the
appended claims.
* * * * *